The present invention relates to an insulation substrate, on which semiconductor chips are soldered. The insulation substrate includes a ceramic core plate (insulation core plate) having foil-shaped copper plates bonded directly to it. The insulation substrate is used in "power modules" or power semiconductor devices such as converters, inverters, switching power supplies, constant voltage constant frequency (CVCF) power supplies, variable voltage variable frequency power supplies, other power supply circuits and control circuits for the various power supplies.
Referring to FIG. 1, a prior art power module has a Ceramic Bonding Copper (CBC) substrate 2 soldered to a metal radiator base 1. CBC substrate 2 includes a ceramic plate 2a sandwiched between a thin foil-shaped upper copper plate 2c and a thin foil-shaped lower copper plate 2b. Each of upper copper plate 2c and lower copper plate 2c is directly bonded to ceramic plate 2a by a direct bonding copper method (the method of bonding copper by using a Cu--O eutectic liquid phase yielded by a reaction of copper and a trace amount of oxygen). Ceramic plate 2a is made, for example, of alumina (Al.sub.2 O.sub.3) or aluminum nitride (AlN). A circuit pattern (thick film circuit pattern) is formed on an upper surface of upper copper plate 2c. A semiconductor chip 3 is mounted on CBC substrate 2.
Curved tips of lead terminals 4 are soldered to the circuit pattern formed on upper copper plate 2c. A terminal block 7 fixes the positions of lead terminals 4. Semiconductor chip 3 is electrically connected to lead terminals 4 by bonding wires 5. A gel resin 9 fills an inner space formed by metal radiator base 1 on the bottom and a resin case 6 on the sides. A sealant resin 8 on top of gel resin 9 closes resin case 6.
In the above described prior art embodiment, problems regarding the radiation of generated heat ensue when CBC substrate 2 is used as a thick film circuit board on which semiconductor chip 3 of a power module is mounted.
Since currents in power modules are larger than other semiconductor circuits, a semiconductor chip 3 in a power module generates a considerable amount of heat when a large amount of current flows through it. The generated heat is conducted by metal radiator base 1 via CBC substrate 2 and radiated from metal radiator base 1. Therefore, the thermal conductivity of CBC substrate 2 is an important factor in determining the current capacity of the semiconductor device.
However, because CBC substrate 2 has a laminate structure (ie. thin foil-shaped upper and lower copper plates 2c and 2b on respective sides of ceramic core plate 2a), its thermal conductivity is relatively low. The specific thermal conductivity values of alumina and aluminum nitride, used as the material of ceramic plate 2a, are listed below.
Alumina: 21 W/m.multidot.k PA1 Aluminum nitride: 180 W/m.multidot.k PA1 Silicon (semiconductor chip): 4.0.times.10.sup.6 /.degree.C. PA1 Alumina: 7.5.times.10.sup.6 /.degree.C. PA1 Copper: 18.0.times.10.sup.6 /.degree.C.
Although aluminum nitride exhibits a thermal conductivity that is far superior to that of alumina, aluminum nitride is much more expensive than alumina and is therefore undesirable for that reason.
To improve the heat radiation of the prior art semiconductor devices, metal radiator plate 1 is sometimes fixed to a heat sink with a bolt or bolts. However, force exerted by the bolts and warping caused by heat exert a bending stress on metal radiator plate 1. The resulting bending stress can deflect metal radiator plate 1 and CBC substrate 2 which is soldered to the metal radiator plate 1. Because the flexural strength and deflection tolerance of ceramics are relatively low compared to metal radiator 1, cracks or cleavages develop in the CBC substrate, thereby reducing its insulative properties.
The flexural strength is approximately 300 MPa for aluminum nitride and 400 MPa for alumina, and the deflection tolerance is approximately 0.2 mm for aluminum nitride and 0.3 mm for alumina. Therefore, aluminum nitride, which has greater thermal conductivity, is weaker than alumina mechanically. Various counter measures should be taken to prevent the development of cracks in the CBC substrate, even though the deviations of the deflection in the heat sink have been estimated to be approximately 100 .mu.m.
It is possible to design CBC substrate 2 with a deflection tolerance of a fraction of the deflection caused in the semiconductor device, to prevent the development of cracks in CBC substrate 2, by dividing CBC substrate 2 into a plurality of substrates. However, the spaces between the independent CBC substrates fixed on metal radiator base 1 conflict with the design goals of the device integration and size reduction. Dividing CBC substrate 2 enlarges the semiconductor device and increases the number of parts and components as well adding cumbersome internal wiring work between the individual CBC substrates. Therefore, using a plurality of thin CBC substrate layers instead of a single larger one is not an effective solution.
Referring to FIG. 2, another counter measure for preventing the development of cracks in CBC substrate 2 includes an elastic bending part 4b formed on the side of a tip part 4a in an inner lead section of lead terminal 4. In addition, the peripheral part of upper copper plate 2c, on which a circuit pattern is formed, is raised above the upper surface of ceramic plate 2a. Since the stress caused in the ceramic plate 2a may be relaxed by the inclusion of the elastic bending part 4b and the floated peripheral part of the copper plate 2c, the development of cracks in the CBC substrate is prevented.
However, this counter measure requires a pressing process for each lead terminal 4 to form the elastic bending part 4b in the inner lead section. This counter measure is not easily applied to the batch assembly of the power modules. In the batch process, lead frames for the power modules are made of many lead terminals 4 connected with one another by a tie bar. The lead terminals are fixed to terminal block 7, the tip parts 4a are soldered, and finally, the tie bar is cut. The pressing process is difficult to incorporate into this batch assembly procedure. In addition to the increased complexity of the batch assembly of lead frames, the manufacturing cost of CBC substrate 2 is also increased since etching and cleaning processes are necessary to float the peripheral part of upper copper plate 2c.